Auto merge of #6829 - ehuss:fix-mutable-borrow, r=alexcrichton
Auto merge of #6829 - ehuss:fix-mutable-borrow, r=alexcrichton

Fix mutable_borrow_reservation_conflict warning.

nightly-2019-04-08 added a more restrictive borrowing rule (rust-lang/rust#58739).

eminence/cargo,tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,rust-lang/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,tsurai/cargo,eminence/cargo,tsurai/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,vojtechkral/cargo,tshepang/cargo,vojtechkral/cargo,tshepang/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,rustyhorde/cargo,tshepang/cargo